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The selection verification of RFID magnetic separators is usually completed in a room temperature laboratory. Attach a magnetic separator to the metal plate, use a fixed model label and reader to measure the reading distance. If the recovery rate reaches the specified value, the plan is approved.
This process implies an assumption: the performance of the magnetic separator remains stable within the service temperature range. Labels in cold chain logistics need to work for hours to days at -30 ° C to -10 ° C; Labels in industrial storage and outdoor environments may be exposed to temperatures between 50 ° C and 80 ° C for extended periods of time. At these temperatures, the magnetic permeability and dielectric constant of the magnetic separator are not at room temperature, and the resonance conditions of the tag antenna change accordingly. At room temperature, the reading distance of the tag may experience batch degradation.
The temperature changes two parameters, not one
The RFID tag antenna is a resonant structure, and its resonant frequency is determined by the geometric dimensions of the antenna and the surrounding dielectric environment. The magnetic separator is attached near the antenna and participates in resonance through two parameters:
Magnetic permeability μ '
Affects the magnetic flux distribution in the near-field of the antenna and changes the equivalent inductance of the antenna. When μ 'changes with temperature, the inductance drifts accordingly.
Dielectric constant ε′
Changing the capacitance distribution around the antenna is equivalent to adding a component to the antenna capacitance. When ε 'changes with temperature, the capacitance drifts accordingly.
The direction of change for the two parameters is not always consistent. The μ 'of ferrite usually decreases with increasing temperature before approaching the Curie temperature, while the ε' of polymer matrix usually increases with increasing temperature (dipole polarization enhancement). One increases the resonant frequency and the other decreases it, and the net effect depends on the relative contribution of the material system and antenna type.
This is the core of the temperature coefficient problem: the thermal behavior of the magnetic separator cannot be summarized by just one "temperature stability" indicator. It is necessary to separately confirm the temperature curves of μ 'and ε', as well as the actual impact of their superposition on the resonance of the target tag antenna.
Cold chain scenario: μ 'variation and label mismatch at low temperatures
The temperature in cold chain scenarios is usually between -30 ° C and -10 ° C. In this range, the μ 'of the ferrite separator may be higher than the room temperature value - the magnetic crystal anisotropy field of some ferrite formulations is enhanced at low temperatures, and μ' increases. The increase of μ 'causes the equivalent inductance of the tag antenna to increase, and the resonant frequency shifts towards lower frequencies.
The superposition effect of low-temperature offset and condensed water
If the tag antenna has been tuned to the target frequency at room temperature (such as 915 MHz in the UHF band), the resonance shift caused by the rise of μ 'at low temperatures may cause the antenna to deviate from the operating frequency band of the reader/writer. The offset depends on the temperature coefficient of μ 'and the thickness of the magnetic separator. Thin magnetic separators have a smaller offset, while thick magnetic separators have a larger offset.
Another variable in the cold chain scenario is condensation water. When removing the label from the cold storage, the surface condensed water film (with an angle of approximately 80) will significantly change the dielectric environment around the antenna, further pulling the resonant frequency. This effect is completely invisible in room temperature testing, but it repeatedly occurs in actual cold chain operations.
High temperature storage: resonant drift dominated by the rise of ε '
In high-temperature storage (50 ° C-80 ° C) scenarios, the ε 'of the polymer matrix increases with temperature, and this effect is usually more significant than the change in μ'. The rise of ε 'increases the antenna capacitance and shifts the resonant frequency towards lower frequencies.
Resonance offset and read distance degradation
For UHF tags, shifting the resonant frequency towards lower frequencies means that the antenna's matching in the target frequency band (such as 860-960 MHz) deteriorates, the backscatter efficiency decreases, and the reading distance shortens. For HF/NFC tags, an increase in ε 'also causes the resonant frequency to deviate from 13.56 MHz, resulting in a decrease in read distance or communication failure.
Irreversible drift after high-temperature aging
High temperature also accelerates the aging of polymer matrix. After long-term high temperature exposure, the cross-linking density and polar group distribution of the matrix change, and the room temperature value of ε 'may also undergo irreversible drift. This means that the selection of magnetic separators for high-temperature scenarios not only requires confirmation of instantaneous performance at high temperatures, but also confirmation of performance retention after high-temperature aging.
Why can't reading distance data at room temperature predict the performance of temperature scenarios
Room temperature reading distance data is a single value measured at a specific temperature. It does not describe the variation curve of reading distance with temperature, nor does it distinguish the contributions of μ 'and ε'. In cold chain or high-temperature scenarios, engineers need a "read distance temperature curve" rather than a normal temperature point.
More importantly, the temperature coefficients of different material systems vary greatly. The temperature coefficient of ferrite separators is different from that of nanocrystalline separators; The ε 'temperature coefficients of silicone rubber matrix and epoxy matrix are also different. Comparing different material systems horizontally using room temperature reading distance data may lead to opposite conclusions in temperature scenarios - schemes with higher reading distance recovery rates at room temperature may perform worse at the target temperature due to a larger temperature coefficient.
Selection and judgment of RFID magnetic separators based on "temperature resonance stability"
Firstly, suppliers are required to provide temperature curves of μ 'and ε' within the target temperature range, rather than just providing single point data at room temperature
The amplitude of the change in μ 'between -30 ° C and 0 ° C should be confirmed for cold chain scenarios; The variation amplitude of ε 'in the range of 50 ° C-80 ° C should be confirmed for high temperature scenarios. If the supplier can only provide room temperature data, the applicability of temperature scenarios cannot be determined.
Secondly, distinguish the contributions of the μ 'temperature coefficient and the ε' temperature coefficient, rather than just looking at the 'comprehensive stability'
The direction of change between the two may be opposite, and the net effect depends on the type of tag antenna. When selecting, it should be confirmed whether the supplier can provide separate temperature data for μ 'and ε', as well as an estimate of the offset of the resonant frequency of the target frequency band after the two are superimposed.
Thirdly, during the sample stage, retest the reading distance using the actual label at the target temperature, rather than releasing it after testing at room temperature
Cold chain scenarios should be tested in low-temperature chambers, while high-temperature scenarios should be tested in constant temperature chambers, using label models and metal substrates consistent with actual applications. The purpose of retesting is to confirm whether the resonance offset is still within the tolerance range of the tag chip.
Fourthly, in high-temperature long-life scenarios, the ε 'drift after high-temperature aging is used as an independent verification term
Long term exposure to high temperatures may cause irreversible changes in the dielectric properties of polymer matrices. When selecting, the supplier should be required to provide data on the changes in ε 'after high-temperature storage (such as 85 ° C/1000 hours) to confirm the resonance stability after aging.
Advanced Institute (Shenzhen) Technology Co., LtdThe RFID magnetic isolation sheet product line covers three frequency bands: LF, HF/NFC, and UHF. It can provide temperature curve data of μ 'and ε' within the target temperature range, as well as dielectric property retention data after high-temperature aging.
The product supports material system matching based on the actual temperature conditions of cold chain or high temperature scenarios, helping engineers accurately predict resonance offset and read distance performance in temperature scenarios. For more technical information, please visithttp://www.avanzado.cn/.